The process control industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. Process variables include pressure, temperature, flow, level, turbidity, density, concentration, chemical composition and other properties. A process variable transmitter can provide an output related to the sensed process variable over a process control loop to a control room, such that the process can be monitored and controlled.
The process control loop can be any configuration of two or more conductors that are capable of passing signals related to process information. For example, the process control loop can be a two-wire, 4-20 mA process control loop. A process variable transmitter coupled to such a process control loop controls the amount of current flowing through the loop such that the current corresponds to the process variable. In some process control loop embodiments, the energization levels can be low enough that even under fault conditions the loop generally will not contain enough electrical energy to generate a spark. This facilitates compliance with the intrinsic safety specification as set forth in the Factory Mutual Approval Standard entitled "Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, and III, Division 1 Hazardous (Classified) Locations," Class Number 3610, published October 1988. Intrinsic safety compliance is particularly relevant in flammable environments because such compliance ensures such low power levels that the possibility of undesirable spark generation is reduced. Low power process variable transmitters can operate on such low energy levels that they can receive all required electrical power from a 4-20 mA process control loop. The process control loop may also have digital signals superimposed on the loop according to a process industry standard protocol such as the HART.RTM. digital protocol.
Low Power Time Domain Reflectometry Radar (LPTDRR) instruments have been used recently to measure the level of products (either liquids or solids) in storage vessels. In Time Domain Reflectometry, electromagnetic energy is transmitted from a source, along a microwave waveguide (also known as a termination), and is reflected at a discontinuity. The travel time of the received energy is based on the media through which it travels as well as the distance traveled. One type of LPTDRR is known as Micropower Impulse Radar (MIR), which was developed by the Lawrence Livermore National Laboratory.
Low power radar level instruments, such as radar level transmitters, that are used in the industrial marketplace see many harsh environmental conditions. In some installations, transmitters can be subject to vibration, and/or extreme temperature swings. Such conditions can adversely affect the physical coupling between the microwave generation circuitry and the waveguide. As the coupling degrades, or is severed, the ability of the transmitter to provide an indication of process product level is adversely affected. Additionally, if the waveguide is submerged in process product, error can be caused in the level output.
As process product level measurement technology advances, there is a need to provide radar level measurement instruments that can identify conditions where accuracy and precision of the level output has degraded. Such an instrument would provide enhanced process control and maintenance because instrument error conditions would be quickly identified and remedied instead of creating erroneous level outputs.